1. Field of the Invention
The present invention relates to optical transmission systems in general and in particular to a delay distortion compensating circuit for use in a laser driven fiber optic transmission system.
2. Description of the Prior Art
The transmission of modulated signals in laser driven fiber optic transmission systems is becoming more and more widespread as the need for transmission bandwidth and high signal-to-noise transmissions increases. For example, more and more cable television systems and other information transmission systems, e.g. telephone systems, which normally transmit vast amounts of information over long distances are using such optical transmission systems.
A typical optical transmission system comprises a laser on its transmitting end and a photodetector on its receiving end. The laser and the photodetector, which may be separated by many kilometers, are coupled by a fiber optic cable.
In operation, modulating signals modulate the light, i.e. optical power output, of the laser. The modulated light then propagates through the fiber optic cable to the photodetector.
A problem encountered in the conventional laser driven fiber optic transmission system is waveform delay distortion caused by phenomena known as laser chirp and chromatic dispersion.
Laser chirp comprises a shift in the frequency/wavelength of the output of the laser as a result of the modulation of the laser. Compared to the wavelength of an unmodulated output from the laser, as the amplitude of the modulation signal increases in a positive direction the wavelength of the output tends to decrease and as the amplitude of the modulation signal increases in a negative direction the wavelength of the output tends to increase.
In the fiber optic cable, this shift in wavelength and a chromatic dispersion of the transmitted laser beam results in a delay distortion comprising an amplitude dependent time delay at the receiving end. For example, in a standard or conventional single mode fiber optic cable, the dispersion of the light having a wavelength of 1310 nanometers (nm) is essentially zero while at a wavelength of 1550 nm the dispersion is approximately 17 picoseconds/nanometer-kilometer (ps/nm-km).